1. Field of the Invention
The present invention relates to an aluminum alloy clad sheet for heat exchangers, for example, for use in automobiles.
2. Description of the Related Art
In general, various aluminum alloy clad sheets that include a brazing layer and a sacrificial layer (an anticorrosive sacrificial layer for a core layer) on one or both sides of a core layer have been used for automotive heat exchangers. Such an aluminum alloy clad sheet for heat exchangers is currently required to have high strength, high corrosion resistance, and a small thickness, for example, 0.3 mm or less to reduce the vehicle weight.
For example, Japanese Unexamined Patent Application Publication No. 11-61306 discloses an aluminum alloy composite sheet for heat exchangers in which, while the Zn and Mn contents in a sacrificial layer (sacrificial anode layer) are specified, the size and distribution of an Al—Mn intermetallic compound in the sacrificial layer are controlled to reduce the corrosion current associated with the anticorrosive effects of the sacrificial layer, thereby increasing the corrosion resistance of the aluminum alloy composite sheet.
More specifically, each side of an aluminum alloy core layer of the aluminum alloy composite sheet is clad with an aluminum alloy sacrificial layer and an Al—Si alloy brazing layer to reduce the corrosion current associated with the anticorrosive effects of the sacrificial layer to 40 μA/cm2 or less. The aluminum alloy of the sacrificial layer contains 1.0% to 6.0% by mass Zn, 0.2% to 2.0% by mass Mn, and the remainder of Al and incidental impurities and contains an Al—Mn intermetallic compound having an average particle size in the range of 0.1 to 0.8 μm at a number density of 2.0×109/mm3 or more. The Al—Si alloy brazing layer contains a predetermined amount of Si.
Japanese Unexamined Patent Application Publication No. 2004-76057 discloses an aluminum alloy clad sheet for heat exchangers in which, while the Mn, Cu, Si, and Fe contents in a core layer and the Zn, Mn, Si, and Fe contents in a sacrificial layer (sacrificial anode layer) are specified, the size and density of a compound in the sacrificial layer are adjusted to control electric potential gradient and corrosion morphology, thereby increasing the corrosion resistance of the aluminum alloy clad sheet.
More specifically, one side of the core layer of the aluminum alloy clad sheet for heat exchangers is clad with an Al—Si brazing layer, and the other side of the core layer is clad with a sacrificial layer. The core layer contains Mn: 0.6% to 2.0% by mass, Cu: 0.3% to 1.0% by mass, Si: 0.3% to 1.2% by mass, Fe: 0.01% to 0.4% by mass, and the remainder of Al and impurities. The sacrificial layer contains Zn: 2.0% to 6.0% by mass, Mn: 0.2% to 1.0% by mass, Si: 0.01% to 0.4% by mass, Fe: 0.01% to 0.3% by mass, and the remainder of Al and impurities. The number of compound particles having a size of 0.1 μm or more in a Mn compound, a Si compound, and a Fe compound in the matrix of the sacrificial layer is 2×106/mm2 or less.
However, existing aluminum alloy clad sheets for heat exchangers have the following problems.
Although the thickness of a sheet for automotive heat exchangers has been reduced, a further reduction in thickness is increasingly required for further reductions in weight, size, and costs. A reduction in the thickness of an aluminum alloy clad sheet for heat exchangers requires high corrosion resistance. Aluminum alloy clad sheets for heat exchangers also require excellent brazeability.
Although corrosion resistance and brazeability have been improved by conventional techniques, there is a demand for development of aluminum alloy clad sheets for heat exchangers having higher corrosion resistance and excellent brazeability to reduce the thickness of the sheets.
In general, to increase the strength of aluminum alloy clad sheets, alloying elements, such as Mn, Fe, Si, and Cu, are added to aluminum alloys. In such aluminum alloy clad sheets, for example, it is difficult to achieve sufficient corrosion resistance only by controlling the Al—Mn intermetallic compound as described in Japanese Unexamined Patent Application Publication No. 11-61306. In particular, under conditions of use where pore corrosion (hereinafter referred to as “pitting corrosion”; resistance to pitting corrosion is referred to as “pitting corrosion resistance”) proceeds, for example, when aluminum alloy clad sheets are used in automotive radiator tubes, pitting (a hole from the inner surface to the outer surface of a tube) may occur within a relatively short time.
Alloying elements, such as Mn, Fe, Si, and Cu, added to aluminum alloy clad sheets form intermetallic compounds, for example, Al—Mn, such as MnAl6, Al12SiMn3, and Al12Si(Mn, Fe)3, Al—Cu, such as Al2Cu, and other intermetallic compounds, such as Al3Fe and Al12Fe3Si, in the aluminum alloy. After brazing heating at 595° C. for 3 minutes, an alloying element, such as Cu, contained in an aluminum alloy of a core layer may diffuse from the core layer and dissolve in the matrix of an aluminum alloy of a sacrificial layer as solid solution or form an intermetallic compound, as described above. The formation of an intermetallic compound is an inevitable phenomenon, for example, in the addition of an alloying element or hot rolling.
Among these intermetallic compounds, Al—Mn, Al—Mn—Si, and Al—Cu intermetallic compounds are the origins of corrosion. More specifically, Al—Mn, Al—Mn—Si, and Al—Cu intermetallic compounds act as cathode sites in an aluminum alloy and accelerate local corrosion in the neighborhoods of the cathode sites, thus accelerating pitting corrosion. In particular, a small number of coarse intermetallic compound produce a small number of pitting corrosion sites originating from the particles, thus concentrating anodic dissolution on the pitting corrosion sites and accelerating pitting.